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Vintage Radio
By RODNEY CHAMPNESS, VK3UG
The impressive AWA Radiola
B32 transistor portable
Australian manufacturers produced several
high-performance transistor radio receivers
during the 1960s – an era that was to herald the
end of local domestic radio manufacturing. One
such set was the AWA Radiola B32 transistor
portable.
Tennant Creek and Katherine. They
transmit at night on 2310kHz, 2325kHz
and 2485kHz and during the day on
4835kHz, 4910kHz and 5025kHz. The
daytime transmissions are not usually
heard in populous areas but some of
the night-time transmissions can be
heard quite clearly in these areas.
Although broadcast-band sets
dominated, some of the transistor
receivers produced by Australian
manufacturers during the 1960s were
multiband sets, which meant that
the ABC’s shortwave inland services
could be heard almost anywhere in
Australia. Of course, those original
One interesting high-performance
receiver from the era is the AWA
Radiola B32 transistor portable.
Produced around 1965, it has a large
195 x 12mm loopstick antenna, followed by a sensitive low-noise radio
frequency (RF) stage which feeds an
autodyne converter. A 2-stage 455kHz
intermediate frequency (IF) amplifier
then feeds a diode detector and three
stages of audio amplification. The
transformer-coupled output stage is
wired in push-pull configuration and
drives an oval loudspeaker measuring
150 x 100mm.
The case is quite substantial for a
transistor receiver and measures 320
x 200 (not including the handle) x
110mm (including the knobs). The
main part of the case is made of
leather, while the front of the set is
metal and this is attached to a plastic
inner frame.
As can be seen in the photographs,
there is quite a bit of room in the back
of the set (to the left). This is used
for the extra features included in the
4-band version of this set, the B33.
Overall, the cabinet is of somewhat
higher quality than some of the downmarket AWA transistor receivers.
Due to its size, the set is quite heavy,
weighing in at 3.3kg without the battery and around 4kg with the battery
in place. It uses the now unobtainable
2761 battery but two D-cell battery
inland services, located at Lyndhurst
(Melbourne) and also near other capital cities, have long since closed down.
In fact, the old Lyndhurst site is now a
housing estate!
However, there are now three stations that have taken over this role and
they are located near Alice Springs,
The AWA B32 transistor portable was a relatively large unit with a leather case
and a metal front panel. This view shows the set after restoration.
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The B32 receiver
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This is the view inside the back of the set, with the modified battery packs in the
foreground. The modified battery packs are necessary because the original 2761
type battery is no longer available.
holders – one for four cells and the
other for two cells – will fit into the
space that the 2761 occupied.
In my case, I removed the battery
plug and wired the battery leads to
the two holders in series to obtain the
required 9V. This arrangement works
well and is the only “non-standard”
work I needed to do in order to get the
set working.
The performance of the receiver can
be enhanced by connecting an antenna
and earth to the bayonet socket at one
end of the receiver. In addition, there
is a socket for an external 9V power
pack and this can be used to extend
the already long battery life of this
set. However, there’s one important
thing to note here: like most transistor receivers using PNP transistors,
this set has a positive chassis. This
means that the sleeve of the socket is
also positive.
Restoring the B32
For some time now, I have been keen
to obtain the 4-band B33 version of this
set and I still am. Even so, I jumped
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at the chance when a B32 model was
offered to me.
The set had quite bad distortion but
I thought that this would be easy to
fix. Well, the fault was easy to find – a
bit of careful probing with the signal
tracer revealed that the audio was fine
up to the loudspeaker. A quick check
with a multimeter then revealed that
one half of the speaker voice-coil was
open circuit.
But where do you get 80-ohm centretapped voice coil speakers? Answer
– you can’t.
I needed to get a close look at the
speaker, which meant that the “works”
had to come out of the cabinet. This initially involves removing three knobs
and the six screws holding the chassis
plate in position. That done, the small
escutcheon around the external power
inlet and the antenna/earth socket are
removed, after which the chassis can
be withdrawn from the cabinet.
Finally, the four nuts holding the
speaker were undone and the speaker
removed. But could I repair it? I could
see the break in one of the voice-coil
leads, so I decided to attempt a repair
using a small soldering iron, some
thin solder and a short length of fine
wire. I was in luck – the repair was
successful.
With the speaker now working
again, the next step was to check the
IF alignment. It needed only a minor
tweak to get it right on the button. I
then checked the front-end alignment
but started getting some strange results. The oscillator was tracking with
the dial calibrations so no work was
needed there. However, I found that
the loopstick antenna needed some
extra turns to peak its performance at
August 2005 95
This view shows the unit with the chassis out of the cabinet, as far as the loudspeaker leads will permit. The loudspeaker is a large oval-shaped unit, which
contributed to the good sound.
(TR2 & C5) adjustments. However,
I couldn’t find anything wrong in
the circuit and all the adjustments
had been done correctly.
Not long after, I had another
reputable brand transistor set to
restore. It too had an RF stage and
it had exactly the same problem,
only worse. Why?
I have come to the conclusion
that the MSP (Manufacturers Special
Products) 3-gang padderless tuning
capacitors have not been cut accurately and so the circuits do not track
correctly. Assuming that the oscillator
gang plates are cut correctly, then it
appears that the RF and antenna capacitors have insufficient capacitance
when the plates are fully in mesh.
By contrast, the Kriesler 11-99
receiver uses a padderless twin tuning gang and the cut on these is near
enough to be perfect. Its tuned circuits
track very accurately and the set is
extremely sensitive. It’s also worth
noting that the B33 doesn’t suffer from
this problem, as the three gang sections
are identical.
That said, the B32 is still a good
performer. It’s just that it could have
been even better.
Cabinet restoration
Fortunately, the cabinet was in very
good condition and only required
wiping over with a soapy cloth. In
addition, a stiff brush was used to get
dust out of the crevices in the front
panel, while the knobs were scrubbed
in soapy water with a nail brush.
The cabinet is in such good order
that it’s apparent that the set hasn’t
been used much. Perhaps the fault
in the speaker occurred quite early
in its life, after which the set was
safely stowed away on a shelf or in a
wardrobe.
Circuit details
A full circuit diagram is pasted onto the inside back cover of the receiver, along
with a diagram showing the dial stringing arrangement and a third diagram
showing the locations of the major components (IF transformers & transistors).
the low-frequency end of the dial.
The antenna coil is wound almost
right along the full length of the loopstick, so it’s not practical to move it to
peak the performance. After adding the
extra turns, the set worked very well at
the low-frequency end of the dial but I
couldn’t reduce the trimmer capacitor
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far enough to peak the performance at
the high-frequency end.
So was the distributed capacitance
across the coil too high? I doubted
this would be the case and decided
to probe further. The components
involved were TR1 and C6. The same
problem occurred with the RF circuit
Fig.1 shows the circuit details of the
B32. It uses eight transistors, all PNP
germanium types, so all voltages are
negative with respect to the chassis
(ie, positive chassis).
The RF stage uses a 2N1637 transistor (VT1) which has automatic
gain control (AGC) applied to it. A
milliamp meter in the collector circuit
displays the signal strength.
VT2 (2N1639) and TR3 function as
an autodyne mixer. Its output signal
is at the intermediate frequency (IF,
455kHz) and this is fed to a top-cousiliconchip.com.au
Fig.1: the circuit uses eight PNP germanium transistors in a conventional superhet configuration. Transistor VT1
functions as an RF stage, while the push-pull output stage (VT7 & VT8) drives an 80-ohm centre-tapped loudspeaker.
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August 2005 97
Photo Gallery: AWA Radiola 46E
to a pair of 2N217S output transistors
operating in push-pull configuration.
The operating bias on the 2N217S
output transistors is stabilised by an
AS2 compensation diode (MR4). This
is necessary because the hotter the
environment, the more current the
output transistors will draw. At the
same time, the compensation diode
also conducts more and the voltage
across it decreases. As a result, the
forward bias on the output transistors
is reduced and so they are automatically “throttled” back.
This means that the standing current drawn by the output transistors
is maintained at about the same level,
regardless of temperature. So MR4
is there to prevent what is known as
“thermal runaway”.
There is no conventional output
transformer. Instead, as mentioned
previously, the output transistors directly-drive an 80-ohm centre-tapped
loudspeaker.
It’s just as well the loudspeaker was
repairable. They’re virtually unobtainable these days and it would have
been quite a lot of work to modify the
audio amplifier stage to suit a different speaker.
Servicing
Manufactured by AWA in 1932, the Radiola 46E TRF console was a
5-valve “little brother” to the larger 55E console featured last month. In
fact, its external appearance was almost identical to the 55E but it featured one less RF stage (and one less knob). The valve line-up was two
235 RF amplifiers, a 224A detector, a 247 audio output stage and a 280
rectifier. Photo: Historical Radio Society of Australia, Inc.
pled bandpass pair of tuned circuits
(TR4 and TR5). The first and second IF
amplifiers both use 2N1638 transistors
(VT3 and VT4), which are neutralised
C22 and C24 (after all, these are solid
state equivalents of triodes)! The IF
stage then drives diode MR3 which
functions as the detector.
The detector output is fed to volume
control RV1 and then to the base of
the first audio stage (VT5, 2N408).
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This stage also acts as an AGC amplifier. The greater the signal, the higher
the current through VT5 for a given
setting of the volume control (ie, the
transistor turns on harder). This in turn
pulls VT1’s emitter more negative and
reduces its gain.
The audio output from VT5 appears
at its collector and is fed to VT6 (another 2N408) via a 4mF capacitor (C39).
It’s output is then transformer-coupled
Unfortunately, this set can be rather
awkward to service. The controls, tuning gang and ferrite rod are all easily
accessed but the PC board can only
be removed by using a large soldering
iron to unsolder it at four points on the
metal mounting plate. In addition, the
RF and oscillator trimmers can only
be adjusted after removing the chassis
from the case.
This is rather a poor idea, considering that it would not have been hard
to use trimmers that are independent
of the gang and positioned with their
adjustment slots easily accessible from
the back.
By the way, once the PC board has
been freed, it’s important to make sure
that it cannot touch the (positive rail)
chassis. Placing paper or a soft cloth
between the board and the metal chassis plate is a worthwhile precaution
against possible damage.
Summary
In summary, there was very little
wrong with the set, the faulty speaker
being the main problem. All in all,
it is a very good set that’s somewhat
SC
spoiled by its tuning gang.
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